Sterically hindered 4-amino-piperidine with a low dimer content, its production and use

ABSTRACT

The invention relates to a process for reducing the dimerization of piperidines of the formula I ##STR1## where R 1  to R 4  are C 1  - to C 6  -alkyl, R 1  and R 2  and/or R 3  and R 4  together are a CH 2  chain having 2 to 5 carbon atoms, which comprises adding to the piperidine from 0.001 to 0.2% by weight of a reducing agent of the formula MXH 4-m  Y m , where M is an alkali metal, NR 4 , where R are identical or different C 1  -C 4  -alkyl groups, or an equivalent of an alkaline earth metal or an equivalent of zinc, X is boron or aluminum, Y is H or CN and m is 0 or 1. The invention also relates to a mixture of piperidines of the formula I, from 0.001 to 0.2% by weight of a reducing agent and from 1 to 1000 ppm of dimers of the piperidines of the formula I, and to the preparation of hindered amine light stabilizers therefrom.

The present invention relates to a process for reducing the dimerizationof piperidines of the formula I ##STR2## where R¹ to R⁴ are C₁ - to C₆-alkyl, R¹ and R² and/or R³ and R⁴ together are a CH₂ chain having 2 to5 carbon atoms. The present invention furthermore relates to a mixturecomprising piperidines of the formula I, from 1 to 1000 ppm of dimers ofthe formula II and from 0.001 to 0.2% by weight of a reducing agent, andto the use thereof for the preparation of HALS compounds.

Sterically hindered 4-aminopiperidines of the formula I are generallyprepared on an industrial scale from acetone or acetone derivatives. Ican be obtained in one step in a catalytic ring-closure reaction in thepresence of ammonia and hydrogen from the compounds ##STR3## (DE 2 412750) or from triacetoneamines of the formula III ##STR4## by reductiveamination in one or two steps, for example catalytically (see, forexample, DE 2 040 975, DE 2 349 962, DE 26 21 870, EP 33 529, EP 42 119,EP 303 279, EP 410 970, EP 611 137, EP 623 585 and DE 42 10 311). Thesterically hindered 4-aminopiperidines prepared industrially aregenerally purified by distillation.

The sterically hindered 4-aminopiperidines of the formula I have avariety of applications. In particular, they are used as intermediatesin the preparation of UV stabilizers for synthetic polymers. The presentinvention furthermore relates to a mixture comprising piperidine of theformula I, from 1 to 1000 ppm of a dimer of the formula II and from0.001 to 0.2% by weight of a reducing agent, and to the use thereof forthe preparation of HALS (hindered amine light stabilizer) compounds.These are usually piperidines of the formula I in which the nitrogenatom of the 4-amino group has been alkylated or acylated (see R.Gachter, H. Muller (editors), Taschenbuch der Kunststoff-additive, CarlHanser Verlag, Munich, 1979; F. Gugumus, Polym. Degrad. Stabil. 44(1994), 299-322).

For many applications, it is important that the piperidine of theformula I has high chemical purity. This is particularly true if thepiperidine of the formula I is employed for the preparation of HALScompounds, since their product quality, and thus also the quality of thestabilized polymers, is crucially dependent on chemical purity. Inparticular, the dimer of the piperidine of the formula I, which iscolorless, is an interfering by-product which forms in unacceptableamounts from the piperidine of the formula I even on storage under aprotective gas in the dark at room temperature.

No processes are known for preventing dimerization of the piperidine.

U.S. Pat. No. 4,316,837 discloses the reduction of 4-aminopiperidylgroups in branched aliphatic alcohols to the corresponding4-aminopiperidyl-containing branched alcohols. The dimer II of thepiperidine I is not disclosed.

DD 266 799 describes purifying piperidines of the formula I by a processin which they are reacted in acetone/water solution with CO₂, theprecipitate is separated off, washed with acetone and then pyrolyzed,and the product is purified by distillation. SU 18 11 527 describesanother purification method for sterically hindered 4-aminopiperidinesof the formula I, in which the unpurified crude product is dissolved inan aprotic solvent and reacted with ethylene glycol, and the reactionproduct is distilled and purified in a number of steps. Both processesare extremely complex and expensive. They do nothing to prevent theinterfering subsequent formation of the dimer.

EP 477 593 discloses that the inherent color of crudeN-alkyldialkanolamines can be improved by adding a metal borohydride anddistilling the mixture under certain conditions in the presence ofwater. However, the N-alkyldialkanolamines are a completely differentclass of substances to the sterically hindered 4-aminopiperidines of thepresent invention. Furthermore, Spec. Chem. 4(2) (1984), 38-41, and U.S.Pat. No. 3,159,276, U.S. Pat. No. 3,207,790 and U.S. Pat. No. 3,222,310disclose that the product color can be improved by addition of sodiumborohydride to ethanolamines, ethyleneamines or aromatic amines. Thispublication likewise does not mention the piperidine of the formula I ora colorless dimer of the formula II.

It is an object of the present invention to prevent or suppress thedimerization of the piperidines of the formula I.

We have found that this object is achieved by the abovementionedprocess, which comprises adding to the piperidine of the formula I from0.001 to 0.2% by weight, based on the piperidine of the formula I, of areducing agent of the formula MXH_(4-m) Y_(m), where M is an alkalimetal, NR₄, where R are identical or different C₁ -C₄ -alkyl groups, oran equivalent of an alkaline earth metal or an equivalent of zinc, X isboron or aluminum, Y is CN and m is 0 or 1. The invention also relatesto the following mixture of substances:

a piperidine of the formula I ##STR5## containing from 1 to 1000 ppm ofa compound of the formula II ##STR6## and from 0.001 to 0.2% by weight,in each case based on the piperidine of the formula I, of a reducingagent. The invention furthermore relates to the use of this mixture forthe preparation of HALS compounds.

Further embodiments of the invention are given in the subclaims.

In the sterically hindered 4-aminopiperidines of the formula I, theradicals R¹, R², R³ and R⁴ are, independently of one another, preferablyC₁ - to C₃ -alkyl, in particular ethyl or methyl, for example methyl.

The reducing agents employed are compounds of the formula MXH_(4-m)Y_(m), where M is an alkali metal, NR₄, where R are identical ordifferent C₁ -C₄ -alkyl groups, or an equivalent of an alkaline earthmetal or an equivalent of zinc, preferably an alkali metal, inparticular sodium or potassium, for example sodium, and X is aluminumor, in particular, boron, Y is CN and m is 1 or in particular 0. Anexample is sodium borohydride. In some cases, (RaO)₂ TiBH₄ or (RaO)₃TiBH₄, where Ra is C₁ -C₄ -alkyl, have also proven successful.

The amount of reducing agent added to the piperidine of the formula I ispreferably from 10 to 1000 ppm, in some cases up to 500 ppm, inparticular up to 200 ppm, based on the piperidine of the formula I. Insome cases, the reducing agent can also be added in amounts of greaterthan 50 ppm, in particular greater than 100 ppm, in each case based onthe piperidine of the formula I.

The method by which it is added is unimportant. The reducing agent canbe added in one or more portions to a vessel containing the piperidineof the formula I and mixed in, for example by stirring. This can becarried out batchwise or continuously, in the latter case for examplealso by metering the reducing agent into a product stream containing orconsisting of the piperidine of the formula I. It is also in principleconceivable to interchange the initially charged and added substances.The temperature and pressure are unimportant, and the reducing agent canbe added during the conventional steps in the preparation of thepiperidine of the formula I, for example during or in particular afterthe distillation step for final purification of the piperidine of theformula I in the preparation process. The reducing agent can be added inpowder form or as a solution, for example in pure piperidine of theformula I.

The substance mixture of claim 5 comprising piperidine of the formula I,dimer of the formula II and reducing agent can be prepared in a simplemanner by first separating crude piperidine of the formula I from dimerof the formula II in the desired manner by distillation. This can becarried out, for example, by distillation since the dimer of the formulaII has a significantly higher boiling point than the piperidine of theformula I. The person skilled in the art can easily determine the dimercontent, for example by gas-chromatographic analysis. The reducing agentis then added as described.

HALS compounds are generally prepared from the novel mixture byalkylation or acylation of the piperidine of the formula I on thenitrogen atom of the 4-amino group in a conventional manner. The novelprocess provides a simple and inexpensive way of preparing pure,sterically hindered 4-aminopiperidines of the formula I which arevirtually free from dimers. This stabilization can take place in asimple manner at room temperature and frequently gives easy-to-handlesolutions which remain free from dimers for at least several weeks.

Advantageous HALS compounds having a reduced by-product content can beprepared therefrom.

EXAMPLES

4208 g of crude 4-amino-2,2,6,6-tetramethylpiperidine(triacetonediamine, TAD) having the composition

85.6% of triacetonediamine (TAD)

9.0% of H₂ O

approx. 0.7% of low boilers

4.7% of medium and high boilers

were rectified in a bench apparatus with a column containing 2.4 m ofSulzer CY packing (approximately 22 theoretical plates, nominal width 43mm) at a reflux ratio of 5:1 and a pressure of from 100 to 40 mbar.After removal of the water in the first runnings at from 43 to 44° C.and 100 mbar, pure TAD fractions having a TAD content of >99.6% wereobtained in the main runnings at 40 mbar and a head temperature of 97 to103° C. (gas chromatography; 30 m capillary column RTX-5 amines);distillation yield: 3022 g (84%).

A pure TAD fraction (TAD content according to GC: 99.7%) was stored andmeasured under identical conditions firstly without addition of sodiumborohydride and secondly with addition of 1000 ppm of sodiumborohydride. The results are shown in the following table:

    ______________________________________                                        Storage                                                                              Composition according to GC (area %)                                   time in                                                                              Without addition of NaBH.sub.4                                                                  With addition of NaBH.sub.4                          weeks  TAD       Dimer II*   TAD    Dimer II*                                 ______________________________________                                        0      99.69     0.02        99.65  0.01                                        23 98.26 0.87 99.64 0.0001 to 0.001                                         ______________________________________                                         *R.sup.1, R.sup.2, R.sup.3 and R.sup.4 = methyl                          

We claim:
 1. A process for reducing the dimerization of piperidines ofthe formula I ##STR7## where R¹ to R⁴ are C₁ - to C₆ -alkyl, R¹ and R²and/or R³ and R⁴ together are a CH₂ chain having 2 to 5 carbon atoms,which comprises adding to the piperidine from 0.001 to 0.2% by weight,based on the piperidine, of a reducing agent of the formula MXH_(4-m)Y_(m), where M is an alkali metal, NR₄, where R are identical ordifferent C₁ -C₄ -alkyl groups, or an equivalent of an alkaline earthmetal or an equivalent of zinc, X is boron or aluminum, Y is CN and m is0 or
 1. 2. A process as claimed in claim 1, wherein R¹, R², R³ and R⁴are C₁ - to C₃ -alkyl.
 3. A process as claimed in claim 2, wherein thecompound of the formula MXH_(4-m) Y_(m) is sodium borohydride.
 4. Amixture comprisingA. a piperidine of the formula I ##STR8## where R¹ toR⁴ are C₁ - to C₆ -alkyl, R¹ and R² and/or R³ and R⁴ together are a CH₂chain having 2 to 5 carbon atoms, B. from 1 to 1000 ppm, based on A, ofa compound of the formula II ##STR9## and C. from 0.001 to 0.2% byweight, based on A, of a reducing agent of the formula MXH_(4-m) Y_(m),where M is an alkali metal, NR₄, where R are identical or different C₁-C₄ -alkyl groups, or an equivalent of an alkaline earth metal or anequivalent of zinc, X is boron or aluminum, Y is CN and m is 0 or
 1. 5.A mixture as claimed in claim 4, where R₁, R², R³ and R⁴ are C₁ - to C₃-alkyl.
 6. A mixture as claimed in claim 4, where the compound of theformula MXH_(4-m) Y_(m) is sodium borohydride.
 7. A method of using amixture as claimed in any of claim 4 for the preparation of HALScompounds.